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The following is an extract from our Lost in Space-Time newsletter. Each month, we hand over the keyboard to a physicist or two to tell you about fascinating ideas from their corner of the universe. You can sign up for Lost in Space-Time for free here.

The fate of the universe once seemed so clear. For the next few billion years, it would continue expanding as it had since the dawn of time. Then, gravity would wrestle back control, beginning a period of contraction. The stars, planets and galaxies would get closer and closer together. Eventually, the universe would crush itself into a gargantuan black hole. Everything would end in seven billion years.听

That鈥檚 what cosmologists were convinced of in the 1960s. But this consensus would be upended by then-unknown PhD student Beatrice Hill Tinsley. She was able to see through the majority view and pioneer some of the first galactic simulations to prove her point.听

When Tinsley started her PhD, much of what scientists thought they knew about the universe鈥檚 fate came from observations made by astronomer Allan Sandage. He used the world鈥檚 largest telescopes to study galaxies a few billion light years from Earth, with one purpose: to map out the universe鈥�s expansion using those galaxies鈥� speeds and distances. 听

Sandage first calculated how fast each galaxy moved. Then, by measuring how bright it appeared in his telescope, he estimated how far away it was. Any shining object appears brighter when closer and dimmer when more remote 鈥� but deriving its precise distance requires knowing how intrinsically bright it is. Otherwise, a bright galaxy far away might be confused with a dim one close by. He worked around this by assuming all his chosen galaxies shone with the same intensity. 听

This turned out to be a major mistake. Nevertheless, Sandage confidently concluded from measurements of the universe鈥�s growth rate that cosmic expansion 鈥�will cease some 3 billion years hence, after which contraction will begin鈥�. The universe, he suggested, would end in seven billion years with a cataclysmic crunch. 听

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Meanwhile, Tinsley, an outstanding undergraduate student in her native New Zealand, was looking for a way to make an impression. In 1963, she travelled to Dallas, Texas to accommodate her husband鈥檚 career. To continue her own research, she commuted 200 miles to Austin, where she was able to pursue astronomy research and complete her PhD thesis in an astoundingly short two years. 听

In a letter to her father outlining the ideas behind her thesis work, Tinsley revealed how she was doubtful that Sandage could be correct, writing that 鈥渢he calculations also depend on what the various galaxies were like at the time the light left them, the light which now gets to the telescopes, and it isn鈥�t necessarily true that they were the same as nearby objects鈥�.听

If ancient galaxies didn鈥�t shine in the same way as galaxies today, Sandage鈥�s projections for the fate of the cosmos were simply wrong. 听

Tinsley鈥檚 thesis tested Sandage鈥檚 constant-brightness assumption using simulations that she designed, coded and analysed. The goal was to calculate how stars are born, live and die 鈥� a tall order, even today.听听

Every star begins as a cloud of gas drifting inside a galaxy. Earlier that century, the physicist James Jeans had already correctly surmised that such clouds are shaped by a delicate interplay between the forces of gravity and pressure, with gravity squashing inwards and pressure pushing outwards. Gravity eventually succeeds in making a tightly packed ball, after which nuclear reactions fire up and turn the inert gas into a shining star. The star then enters its adult life, but it continues to change in colour and brightness over time; once it exhausts its nuclear fuel, it may die in a dramatic explosion. The brightest stars only live for a few million years 鈥� the blink of an eye in terms of cosmic history.听

Knowing the stellar life cycle in abstract is not the same as being able to calculate it with precision. But Tinsley wisely didn鈥�t expect her simulation to directly grapple with any of this. Instead of attempting the detailed calculation of how stars are made from individual clouds, she instructed the computer so that, averaged over an entire galaxy, gas turned into stars at a steady, slow rate that could be specified and adjusted by hand. She cobbled together the way that individual stars shine, age and die from existing pen-and-paper calculations; the computer just needed to add up the effects of all the stars formed during the lifespan of each simulated galaxy. 听

Despite their simplicity, these simulations were sufficiently powerful to prove Sandage wrong. Whatever Tinsley tried, there was no way she could produce galaxies that shone with the same brightness over their entire lives. Keeping the galaxies steady would require forming new stars at just the right rate to replace dying ones. However, this would have implied stars that were a different colour from the ones actually seen through telescopes. Tinsley wrote in her thesis that understanding the origin and fate of the universe 鈥�now appears harder than was previously thought, because of the effects of galactic evolution鈥�.听

The masterstroke in Tinsley鈥�s work is that her simulations didn鈥�t provide a final answer about how the galaxies formed and changed over time 鈥� and that鈥檚 OK. She wasn鈥�t concerned with obtaining a single, correct result; she knew that wouldn鈥�t be possible, given all the complexities involved. Instead, she showed that Sandage鈥�s assumption of unchanging galaxies was untenable. 听

Sandage鈥檚 friends say he was deeply upset. In his mind, an upstart was unfairly attempting to destroy his research. In a 1967 lecture in Oxford, he asserted that Tinsley鈥�s claims were 鈥�spurious鈥�. But Tinsley knew she was right and responded with an even more detailed technical paper, showing that Sandage was the one in error. Over time, her simulations and powerful arguments won over cosmologists. Today we believe the very opposite of Sandage鈥檚 claim: the universe will, in fact, expand forever, as far as anyone can tell.听

A good lesson from Tinsley鈥檚 work is that simulations can still upend our ideas about the universe without being perfectly accurate. Today鈥檚 ultra-sophisticated supercomputers can deliver apparently photo-realistic results, but in truth there are still vast mismatches between real galaxies and our human replicas. 听

We just don鈥檛 have the computational power to get everything right. During my PhD in the mid-2000s, this revelation left me bewildered. I was using simulations to understand the effects of dark matter and dark energy 鈥� postulated invisible substances that sculpt visible galaxies through gravity. 听

At first I had been excited by the important claims being made on the back of such simulations, but as I became aware of their underlying weaknesses I became disillusioned. 听I still have a strange little video that I made on my Nokia camera phone spoofing an oily sales pitch for simulations, making purposefully outrageous, grandiose claims. I must have been at a particularly low point. 听

It is true that modern simulations can easily be oversold. The better they look when , the greater the risk that we brush aside their deficiencies. But if only I had started by reading Tinsley, I would have spared myself some angst. It takes care and expertise, but in the decades since her untimely death in 1981, researchers have repeatedly shown that simulations, even pretty rudimentary ones, can reveal surprising truths about our universe.听听

by Andrew Pontzen is out now听